Recently, there has been a rapid growth in demand for a granular potassium fluorotantalate crystal and a high-purity potassium fluoroniobate crystal as materials for obtaining tantalum particles or niobium particles used for producing an anode of a condenser, a tantalum condenser or a niobium condenser. In this case, a high-purity potassium fluorotantalate crystal or a high-purity potassium fluoroniobate crystal is used as tantalum particles or niobium particles obtained by reduction of the crystal, for example, by contact-reacting it with fumes of metallic sodium.
Accordingly, the interface area of the contact reaction with metallic sodium fumes is a factor of determining the reduction rate when the crystal is reduced to tantalum particles or niobium particles. In general, a potassium fluorotantalate crystal or a potassium fluoroniobate crystal is granular. Thus, the finer the particles, the larger the interface area of the contact reaction is allowed to be. This seems to preferably increase the reduction rate as well. As a result, there has been required the particle size of a potassium fluorotantalate crystal being 4 mm or smaller in order to attain a reduction rate of a minimum requirement industrially needed.
On the other hand, contact with metallic sodium fumes is carried out at a high temperature, and therefore containing crystals with a particle size of 0.15 mm or less in quantity effects sintering of tantalum particles or niobium particles reduced during reduction treatment. In addition, tantalum particles or niobium particles used in production of a high capacity condenser need to be fine. However, when a high-purity potassium fluorotantalate crystal or a high-purity potassium fluoroniobate crystal, the raw material thereof, is too fine, it cannot be obtained as fine metallic particles due to the occurrence of sintering during the reduction to metallic tantalum or metallic niobium. Further, although the same can be said of general metallic particulates, fine metallic particles are highly flammable and thus an easily scattering level of fine particles is not preferable from the viewpoint of operational safety as well.
Thus, sintering of tantalum particles or niobium particles makes it impossible to ensure uniform dispersibility when they are used for condensers, failing in acquisition of good condenser performance. Accordingly, the industry has requested that a potassium fluorotantalate crystal or a potassium fluoroniobate crystal has a particle size of 4 mm or less and the number of particles with a particle size of 0.15 mm or less is to be decreased as much as possible. On the contrary, when a high-purity potassium fluorotantalate crystal is manufactured by a present method, a potassium fluorotantalate crystal with a particle size of 0.15 mm or less accounts on average for 42% or more of the total weight.
In such a case, generally when the particle sizes of powders are tried to be simply divided, physical methods for particle classification such as air classification and sieve classification by repetition can easily attain a target range of particle sizes.
However, general physical methods for particle classification can hardly be applied repeatedly to a potassium fluorotantalate crystal or a potassium fluoroniobate crystal. In other words, these crystal particles are brittle, and thus a plurality of applications of a physical method for particle classification creates a factor of significantly decreasing the yield of a product with a target crystal particle size inasmuch as crystal particles collide to each other in a particle classification step and are likely to be crushed into fine particles. In addition, application of a physical method for particle classification mixes a variety of impurities as contamination in a particle classification step, which effects a factor of lowering purity. This factor is avoided not only in a condenser application, but in any applications.
Hence, there has been required a manufacturing method that can produce a high-purity potassium fluorotantalate crystal or a high-purity potassium fluoroniobate crystal of a crystal particle size distribution matched with a level demanded by the market in steps of manufacturing a potassium fluorotantalate crystal or a potassium fluoroniobate crystal without utilizing a physical method for particle classification.